How to join 2 overlapping planes
It is likely that there are easier ways to achieve the effect you want. However, here's how you can combine the two planes:

1 - Select both of the planes and join them (ControlJ). The two planes are now one object, but we need to take care of the overlapping ends.
2 - Enter edit mode (Tab)
3 - Select the vertex editing option:

4 - Move the two inner points towards the other planes:

5 - Select the two vertices and merge them (AltM + Enter)
6 - Merge the outer corner vertices the same way.

You should now have this:



Note that merging the planes in this way is a bit tedious. Consider using another technique to model your object that does not require joining.